The invention pertains to an alternating light photometer, especially a single-beam photometer, in which a beam which is periodically interrupted by means of a chopper disk rotated over a motor. The interrupted beam is incident on a photoelectric transducer, the corresponding output signal of which is amplified and conducted to a display device, in conjunction with which the chopper disk simultaneously and alternately releases an illuminating beam for one of two photosensitive elements of a switching circuit, by means of which switches creating two differential circuit paths can be alternately controlled.
A photometer of the aforementioned type is described in the German Pat. No. 2.020.387. The commutation between two current paths serves, in conventional photometers, to separately facilitate the (1) intensity of the reference beam on the one hand, and (2) the measuring beam in order to effect a relative measurement of the intensity of the measuring beam in contrast to that of the reference beam. Therefore, in conjunction with a conventional photometer, there is provided a display device for the measuring beam and a display device for the comparison beam which may be alternately controlled.
In the practice of the invention, the procedure is basically carried out in such a manner that the measuring beam is conducted to a photoelectric transducer, for instance a secondary electron multiplier, over a chopper disk for the production of differential beam-intensity. The output signal of the transducer is then composed of the so-called "dark-current signal" and a signal which is dependent on the intensity of the incident radiation. In so doing, it must be considered that the dark-current continually changes. This change of the dark-current is designated as noise.
In conjunction with constant-light photometers, the difficulty arises that the dark-current, which is temperature-sensitive, must be compensated, for which purpose a manual zero-point balance is undertaken in most cases. The result of this is, that very long setting periods are required. A greater problem than the foregoing is, however, when using digital display devices, the low-frequency noise, which is expressed in a slow fluctuation of the display. In conjunction with digital display, this can lead, for instance, to a regular change in the digit position last displayed.
In conjunction with alternating light photometers to which the one according to the invention also belongs, an alternating-voltage is amplified, as a consequence of which the direct-current portion is eliminated. Further, in such types of photometers, the influence of false light, which is incident for instance through an ocular, can be disregarded, because the false light is no longer subjected to chopping by the chopper. With the conventional alternating light photometers, what is now done is, that the alternating-current voltage output signal of the photoelectric transducer, for instance of a photomultiplier, is selectively amplified.
The known alternating light photometers, however, have a number of deficiencies. For one thing, the rotational speed of the motor serving for the drive of the chopper disk, although involving considerable expenditure and effort, does not make it feasible to maintain it sufficiently constant. The consequence of this is, that an amplifier with a proportionately large band width must be used which in turn leads to the fact that the noise signal, especially a low-frequency noise signal is, simultaneously amplified. Further, it has been shown that the long-term stability of the amplifiers customarily used is not good. The tuning which is required from time to time, is complicated and expensive.
Furthermore, in conjunction with the usual photometers which function according to the alternating light method, it is required to rectify the alternating-current voltage before the display. Any rectification, however, again inevitably leads, in turn, to errors and requires a corresponding expenditure. For the elimination of the errors, it has indeed already been proposed to provide a phase-sensitive rectification in German Pat. No. 65.468. This type of rectification, however, is expensive and requires considerable effort.